Capillary gas splitting device

ABSTRACT

A capillary gas-splitting device incorporating a gas-receiving fitting provided at one end with a cylindrical bore and, leading therefrom, a pair of gas outlet tubes between which the entrant gas is split in a ratio which remains constant despite varying gas inlet pressures and flow rates. The first of said tubes incorporates a relatively short section of capillary tubing at any position along its length, and it opens into the bore adjacent the bottom thereof. The second of said tubes is of capillary size and extends into the bore with its extremity running axially thereof for opening against the incoming gas. Though admitted under pressure, the gas is discharged from each tube at atmospheric pressure, and the ratio of the flow rate of incoming gas passing through the one tube as compared with that through the other is directly proportional to the respective capillary lengths in the two tubes. The capillary tube is adapted at its gas-entrant end to receive lengths of wire of extremely small diameter whereby the split ratio of gases passing through the two tubes may be changed as desired. The exit end of the capillary tube may be provided with a fitting adapted to receive nitrogen or other inert gases and to discharge the same in an annular fashion about the end of the capillary tube for even mixing with the gas stream discharged therefrom.

United States Patent Robert G. Brownlee East Palo Alto. Calif.

[72} Inventor 2|1 Appl. No 856,713

[22] Filed Sept. I0. 1969 [45] Patented July 6, 1971 [73] Assignee Stanford Research Institute Menlo Park, Calif.

[54] CAPILLARY GAS SPLI'ITING DEVICE Primary Examiner-Alan Cohan Assistant Examiner-Michael O. Sturm Anorneys--D. J. De Witt and Gregg & Hendricson ABSTRACT: A capillary gas-splitting device incorporating a gas-receiving fitting provided at one end with a cylindrical bore and, leading therefrom, a pair ofgas outlet tubes between which the entrant gas is split in a ratio which remains constant despite varying gas inlet pressures and flow rates. The first of said tubes incorporates a relatively short section of capillary tubing at any position along its length. and it opens into the bore adjacent the bottom thereof. The second of said tubes is of capillary size and extends into the bore with its extremity running axially thereof for opening against the incoming gas. Though admitted under pressure, the gas is discharged from each tube at atmospheric pressure, and the ratio of the flow rate of incoming gas passing through the one tube as compared with that through the other is directly proportional to the respective capillary lengths in the two tubes. The capillary tube is adapted at its gas-entrant end to receive lengths of wire of extremely small diameter whereby the split ratio of gases passing through the two tubes may be changed as desired. The exit end of the capillary tube may be provided with a fitting adapted to receive nitrogen or other inert gases and to discharge the same in an annular fashion about the end of the capillary tube for even mixing with the gas stream discharged therefrom.

CAPILLARY GAS SPLITTING DEVICE BACKGROUND OF TH E INVENTION The device of the present invention is adapted to receive a gaseous effluent from any suitable source such, for example, as that from a vapor phase chromatographic instrument, and to then divide the received gases into two separate streams. The smaller of the two streams can be supplied to a detector, while the balance can go to a sample collection device. While a number of splitting devices are available for this purpose, they suffer one or more operating disadvantages. For example, many of said units can provide a given split ratio only when the device is provided with gas at a predetermined flow rate. Others have a relatively high internal, or dead volume and suffer from loss in resolution and changes in retention time at low flow rates. Again, many of the devices hitherto available engender turbulence of the entrant gasstream at the point of split and thereby set up a variable flow pattern even when gas is supplied to the. splitter at a constant flowrate. It is an object of this invention to overcome these and other disadvantages by providing a unit wherein the split ratio is constant for any rate or pressure of gas flow to the unit and which is characterized by extremely low internal volumes and residence times.

SUMMARY OF THE INVENTION The present invention rests on the discovery that the above and other objects can be achieved by the provision of a gas splitting apparatus comprising a fitting provided at its gas-entrant end with an axial bore adapted to receive a gas stream from a source of supply to which the fitting may be connected, said apparatus having two tubes communicating with said bore and leading from the fitting which split or divide the gas stream and transport the same in a controlled constant ratio of one stream to the other. One of said tubes is capillary in nature while the other has a short capillary section therein, and the flow rate of gas carried by each tube is directly proportional to the respective capillary lengths in the two tubes.

The capillary tube, which is relatively long as compared to the section of capillary tubing forming a part of the second tube, carries a minor percentage of the gases entering the fitting, and the stream passing therethrough is adapted to be connected to a detector instrument for analysis of the stream components. This tube extends through the fitting and projects into the bore thereof, with the terminal portions of the tube within the fitting being aligned with the longitudinal axis of the bore to permit said tube to receive its portion of the gas from the center of the incoming gas stream. The balance of the stream flows smoothly past the entrance to the capillary tube for discharge near the bottom of the bore through the other tube which incorporates a short capillary section. This structure obviates all eddying of the incoming gases and ensures that a representative sample is continuously passed through the capillary tube.

As indicated above, the tube incorporating the short capillary section opens at or near the bottom of the bore, the receiving end of the tube being preferably flush with the bore wall. The capillary section of this tube may be positioned anywhere along its length, though it proves most convenient to mount the said section at the downstream end of this tube by means of a suitable collar structure or the like. While any desired ratio may be employed as regards the respective lengths of the capillary section in this tube and the length of the capillary tube per se, it is preferred that this section have a length which is about one-fifth to one-twentieth that of the length of the capillary tubing. Thus, in one embodiment of theinvention the capillary tube is approximately 6 inches long, while the capillary insert in the other tube is approximately one-half inch long. In this case the capillary tube carries approximately 8 to 10 percent of the total volume ofgas supplied to the fitting.

It is a feature of the present'inve'ntion that since one of the tubes is capillary in nature while the other incorporates a capillary section, the relative volumes of gases carried by the tubes remain the same no matter what the pressure of the incoming gas stream. Moreover, while the gas normally enters the fitting under moderately elevated pressures, e.g., 0.5 to 20 p.s.i.g., the gas exits from each of the tubes at atmospheric pressure.

It forms an additional feature of the present invention that while the ratio of the gas flow rate in the one tube to that in the other is a function of the respective capillary lengths, as discussed above, this ratio may be varied at will by the operator as he inserts lengths of fine wire into the gas-receiving end of the capillary tube. This insertion can readily be made inasmuch as the projecting mouth of the tube extends axially within the fitting and access thereto can readily be had by uncoupling the fitting from its gas supply source. The fine wire inserted in this fashion preferably has a length such as to extend substantially the full length of the capillary tube. Wire inserts of this character increase resistance to flow of the gas carried by the capillary tube, thereby establishing a new, albeit constant split ratio of the entering gas despite changes in the physical or other characteristics of the incoming stream to the fitting.

In one embodiment of this invention, provision is made to mix the gas exiting from the capillary tube with another gas (e.g., nitrogen) in such a fashion that no eddying or turbulence occurs at the point of mixing. Reinforcing the sample stream discharged from the capillary tube in this fashion prevents peak broadening in the detector to which this stream is conventionally supplied, while also increasing detector sensitivity. The mixing device employed for this purpose com-. prises a hollow cylindrical fitting which encloses the outer end portion of the capillary tube and provides an annular space thereabout, said fitting having an inlet port adjacent its base through which the added nitrogen or other gas can be supplied. In this embodiment, the capillary tube passes through the base end of the fitting, to which it is secured, and then extends axially into the interior of the fitting past the inlet port provided for the added gas. The capillary tube terminates short of the other end of the fitting which is provided with an exit port for the combined gas stream.

DESCRIPTION OF PREFERRED EMBODIMENTS For a better understanding of this invention, reference may be had to the following description when read in conjunction with the figures of the accompanying drawing wherein:

FIG. 1 is a view in section of a gas-splitting device embodying features of this invention; and

FIG. 2 is a schematic view of an instrument array in which the apparatus hereof is adapted to form a part.

Referring now to FIG. 1, there is shown a gas-receiving fitting 10 provided with a cylindrical bore 11 and a counterbore 12. The neck of the fitting which surrounds said counterbore is externally threaded at 13 to receive a nut 14 as the fitting is connected to an incoming gas supply line 15. A gastight connection between this line and fitting 10 is made by using a suitable bushing 16 composed of Teflon or the like, said bushing engaging about line 15 and having a conical exterior surface which fits against a mating surface within counterbore l2. Tightening of nut 14 compresses the bushing 16 about the members engaged thereby and effects the desired gastight seal.

A capillary tube 20 is passed through an opening 21 cut into the body of fitting 10 and communicating with bore 11, the tube being soldered into place to prevent passage of gas past the tube through the opening. The upstream, terminal portion 22 of tube 20 extends into counterbore l2 and is axially aligned therewith. As shown in FIG. 1, this brings the open mouth of tube 20 into the center of the incoming gas stream in line 15, thereby essentially avoiding all turbulence or eddying of said gas stream as a portion thereof flows into tube 20 while the balance flows down into the'bore for exit through a noncapillary tube 30. The latter is sealed into place within an opening 31 cut in fitting l and opens adjacent the bottom of the bore 11 to permit smooth exit of that (major) portion of the incoming vapors which are not discharged through the smaller, capillary tube 20.

Tube 30 is provided, in a midportion thereof, with a relatively short section of capillary tubing 32. In the embodiment here shown, an opening 33 having the same internal diameter as that in tube 20 is cut in a tube section which serves to join adjacent lengths of the tube 30. The length of this capillary opening 33, as compared with the length of capillary tube 20, determines the ratio into which the incoming gas stream in line 15 is split between tubes and 30. Moreover, each of said tubes having at least a section of capillary tubing, reduces the pressure of any incoming gases admitted at elevated pressures and discharges the same at atmospheric pressures.

The split ratio of the gases between tubes 20 and remains constant no matter what the rate or the pressure at which gases are supplied to the unit through line 15. However, a new, enhanced ratio can be established by partially obstructing the capillary passage in tube 20 by inserting a length of fine wire therein, as indicated at 40. This wire can most conveniently be inserted into the tube at its end 22 as fitting 10 is disengaged from the upstream assemblage, and recovery of the wire from the tube is facilitated by bending the end of the wire as shown in the drawing. By suing wires of varying diameter, the split ratio can readily be increased from one of 10 to I (wherein tube 20 is unrestricted) to one of 100 to l or even higher.

The exit end of tube 20 can be directly connected with any desired instrument. However, in the preferred embodiment illustrated in FIG. 1, the exit terminal portion 23 of the tube is provided with a fitting 50 which encloses the tube in annular fashion. A tube 60, adapted to carry nitrogen or other inert gas opens into the annular space 61 between the tube extremity 23 and the inner wall of the fitting. Gas discharged from tube 60 mixes smoothly with that discharged from the end of the capillary tube, with the combined gas stream then flowing to an appropriate instrument via a line end of fitting 50.

FIG. 2 of the drawing shows in a schematic fashion the manner in which the present apparatus is adapted to be employed with analytical equipment. The incoming sample of gas, usually carried by helium, first enters a chromatography unit 75. The vaporous exit stream from said unit then passes to the fitting 10 of the present apparatus, with the stream therefrom being divided between the capillary line 20, and the noncapillary line 30 provided with a short section of capillary tubing (not shown). The capillary tube 20 discharges into the annular fitting 50 into which nitrogen or other gas is also supplied via line 60. The combined stream from fitting 50 then passes to a suitable detector instrument 76 of the flame or other type, while the vapors in line 30 are condensed in a sample collection apparatus 77.

lclaim:

l. A gas splitter comprising a fitting provided at one end with an axial bore adapted to receive a gas stream;

a capillary tube leading through the fitting and extending into said bore with the terminal portion of the tube running axially of the bore; and

a noncapillary tube provided with a relatively short section of capillary tubing at some portion along its length, said tube opening into the bore adjacent the bottom thereof.

2. The gas splitter device of claim 1 wherein the capillary space in the capillary tube is partially restricted by a wire which is removably inserted into the tube from the end which extends into the bore of the fitting.

3. The gas splitter device of claim I wherein the exit terminal portion of the capillary tube is provided with a fitting which encloses the tube in a spaced annular fashion and is adapted to receive a gas and to discharge the same about the end of the capillary tube for mixing with the gas stream discharged therefrom, and in the same direction of flow.

62 secured to the open 

1. A gas splitter comprising a fitting provided at one end with an axial bore adapted to receive a gas stream; a capillary tube leading through the fitting and extending into said bore with the terminal portion of the tube running axially of the bore; and a noncapillary tube provided with a relatively short section of capillary tubing at some portion along its length, said tube opening into the bore adjacent the bottom thereof.
 2. The gas splitter device of claim 1 wherein the capillary space in the capillary tube is partially restricted by a wire which is removably inserted into the tube from the end which extends into the bore of the fitting.
 3. The gas splitter device of claim 1 wherein the exit terminal portion of the capillary tube is provided with a fitting which encloses the tube in a spaced annular fashion and is adapted to receive a gas and to discharge the same about the end of the capillary tube for mixing with the gas stream discharged therefrom, and in the same direction of flow. 